1. Technical Field of the Invention
The present invention relates to a solid fuel burner for burning a solid fuel fed by gas flow, particularly to a solid fuel burner, a solid fuel burner combustion method, a combustion apparatus (combustion apparatus) equipped with solid fuel burners and an operation method of the combustion apparatus, suitable for suspended combustion of such fuels as wood, peat and coal, containing much water and volatile matter by pulverizing and feeding them by gas.
2. Description of Prior Art
Since fuel such as wood, peat and coal having a low coalification rank, represented by brown coal and lignite, contains much volatile matter, in the atmosphere of air, such a fuel tends to undergo spontaneous ignition in the process of storage, pulverization and feed, and is not easy to handle, as compared to the bituminous coal. To prevent spontaneous ignition, when such a fuel is pulverized and burnt, the gas mixture of air and exhaust combustion gas with oxygen concentration reduced may be used as a gas for carrying the fuel. The exhaust combustion gas serves to reduce the oxygen concentration around the fuel and to control the oxidation reaction (combustion) of the fuel, thereby avoiding spontaneous ignition. Further, the exhaust combustion gas also serves to remove water content in the fuel by the potential heat.
However, when the fuel carried by the carrier gas is jetted from the solid fuel burner, the oxidation reaction of this fuel is restricted by the oxygen concentration around the fuel, and the combustion speed is low as compared to the cases where the fuel is carried by air. Normally, after having mixed with the air jetted from the air nozzle, the fuel is subjected to active oxidation reaction. Therefore, combustion speed is determined by the speed of mixing with air. Thus, fuel switching time is longer than that when the fuel is carried by air. This will lead to increase in the amount of the unburnt fuel at the outlet of the combustion apparatus, i.e. the furnace
Further, flame temperature is low since combustion is slow. This makes it difficult to utilize the reduction reaction of the nitrogen oxides (NOx) into nitrogen, which becomes active in a reducing atmosphere of high temperature (about 1000° C. or higher). The NOx at the outlet of the furnace tends to be greater in amount than when the fuel is carried by air.
One of the ways to expedite the ignition of the fuel carried by the carrier gas is to increase the oxygen concentration of the fuel carrier gas in the vicinity of the fuel nozzle outlet. For example, proposed is a structure for installation of an additional air nozzle outside the fuel nozzle or at the center of the nozzle. The additional air nozzle promotes mixing of the fuel with air at the fuel nozzle outlet.
If the air from the additional air nozzle is jetted in the form of fuel jet, viz., in parallel to the fuel and its carrier gas as in the case of the aforementioned prior art examples, mixing between the fuel jet and additional air will be slow because there is a small difference in flow velocity between the fuel jet and the air jet from the additional air nozzle.
Normally, the distance from the additional air nozzle outlet to the fuel nozzle outlet is 1 meter or less. Since the velocity of the fuel jet is about 12 meters per second or higher, time of mixing between the fuel particle and additional air in the fuel nozzle is as short as about 0.1 second or less. If the additional air is jetted in parallel to the fuel particle, mixing will be insufficient.
In the meantime, when the additional air nozzle is arranged upstream of the fuel nozzle in order to prolong the time of mixing between the fuel particle and additional air in the fuel nozzle, so-called a back fire may occur, wherein the fuel is fired inside the fuel nozzle if the oxygen concentration is increased. Thus, the distance from the additional air nozzle outlet to the fuel nozzle outlet cannot be increased.
To solve such problems, the present inventors have proposed a structure wherein an additional air nozzle is arranged in the fuel nozzle, and air is jetted from the additional air nozzle in the direction almost perpendicular to the fuel jet running through the fuel nozzle. In another structure proposed by the same present inventors, an additional air nozzle and a separator that separates the flow path are arranged in the fuel nozzle, and the outlet of the additional air nozzle overlaps with the separator, when viewed from the direction perpendicular to the burner shaft. (See Patent Document 1, for example).
Patent Document 1: Japanese Application Patent Laid-open Publications No. 2003-240227 (JP 2003-240227 A) (FIGS. 1 through 4 on pp. 7 through 9.)